The Beatles legend admits he doesn't understand the key to his own success.

He said: ''People used to say to The Beatles in America, 'Who writes the words and who writes the music?' And we'd say, 'Both of us.' 'So what's your formula?' We'd go, 'Well if we had one, we'd bottle it but then probably break the bottle and lose the formula.' You know, you don't want a formula. I don't ever want to find out what I'm doing.''